Mechanical seals suitable for this purpose may comprise a stationary seal element sealed in the housing and a rotary seal element sealed on the shaft, each seal element being provided with opposing annular seal faces surrounding the shaft and urged against one another to form a running seal.
Nevertheless, what ever type of seal is employed it is desirable in some cases to provide a backup seal in the event that the main seal should fail. This is the case where volatile, toxic, flammable etc. fluid products are contained in the housing to be sealed.
A very satisfactory arrangement provides a backup seal comprising a seal of the same type as the main seal and mounted in tandem with the main seal in an extension of the housing. The space defined by the shaft, housing and two seals may be supplied with a circulating barrier fluid which serves several functions. Firstly the barrier fluid serves to cool and lubricate the seal faces and to remove impurities from the region. Secondly if the main seal should fail the increased pressure of the barrier fluid gives a warning of this fact and the barrier fluid serves to dilute and remove the leaking product.
Such a system suffers the disadvantage that it is expensive in terms of the extra equipment that must be provided. It has been suggested to run such tandem seals dry, that is, without a barrier fluid but this means that the backup seal receives no liquid lubrication and hence wears quickly. Thus it has also been suggested in, for instance, German published patent application No. 2659880 (Flexibox) to provide a backup seal comprising a stationary seal element sealed in the housing and a rotary seal element sealed on the shaft, each seal element being provided with opposing annular seal faces surrounding the shaft and being adapted to be urged toward one another to form a running seal therebetween substantially only by the pressure of fluid product leaking past the main seal should that seal fail.
Thus while there is no leak past the main seal there is no significant pressure urging the seal faces together and hence they do not wear. If the main seal should fail however then the increased fluid pressure now acting on the seal elements is arranged to close the seal elements together to form the secondary seal and prevent the product from leaking.
In such an arrangement however it is often found that before the hydraulic pressure can close the seal a quantity of the product leaks through the seal and if the product is particularly toxic, flammable etc then this temporary leak may be unacceptable.
German published patent application No. 2844945 (Burgmann) suggests the provision of a bush on the stationary seal element around the shaft. Should the main seal be broken the fluid product must leak past the bush before it can leak between the seal faces. However because the bush is a relatively close fit on the shaft, the pressure builds up behind the bush and tends to close the seal elements together before significant leakage past the bush occurs and certainly before any appreciable leakage through the seal element begins.
However, because the bush is fixed in the housing it cannot have a fine clearance on the shaft because of the danger of shaft pickup. Thus to get an adequate pressure drop along it the bush must be long. Moreover although a large hydraulic force is initially desirable to close the seal elements together quickly, once they are closed such a large force is not desirable at all because the seal faces would wear too rapidly.